31 Jan
2014
31 Jan
'14
11:29 a.m.
I am sure, a BCCH is a BCCH, this does not hop at all, and the frequency
hopping carriers do not hop some KHz, but in a sequence over a fixed scheme
of normal ARFCNs. There is no free frequency choice in GSM, hopping or not.
Ralph.
From: Nikos Balkanas [mailto:nikos.balkanas@eyeonix.com]
Sent: Friday, January 31, 2014 12:01 PM
To: Ralph A. Schmid, dk5ras
Cc: Richard Koch; osmocom-sdr(a)lists.osmocom.org
Subject: Re: kalibrate-rtl results...
On Fri, Jan 31, 2014 at 12:47 PM, Ralph A. Schmid, dk5ras <ralph(a)schmid.xxx>
wrote:
GSM does not hop around inside its channel, this is wrong, the frequency is
+- some Hz accurate, at least in normal western European commercial GSM
networks.
Are you sure about that? Check it out:
https://svn.berlin.ccc.de/projects/airprobe/wiki/A
Legend of first picture. If still in doubt that *most" GSM carriers are
hoppers, use rtl_power to scan your region ;-)
Nikos
Ralph.
From: osmocom-sdr-bounces(a)lists.osmocom.org
[mailto:osmocom-sdr-bounces@lists.osmocom.org] On Behalf Of Nikos Balkanas
Sent: Thursday, January 30, 2014 7:35 PM
To: Richard Koch
Cc: osmocom-sdr(a)lists.osmocom.org
Subject: Re: kalibrate-rtl results...
On Thu, Jan 30, 2014 at 8:21 PM, Richard Koch <n1gp(a)hotmail.com> wrote:
I'm attempting to calibrate my RTL dongles. I notice they are
all about 2-4khz off if I compare against a known broadcast
station's frequency.
I am trying out kalibrate-rtl and am getting varied results.
I am in the US, so I did a 'kal -s EGSM -g 20' and got the following:
chan: 991 (928.4MHz + 26.986kHz) power: 448020.99
chan: 1002 (930.6MHz - 5.631kHz) power: 320357.07
chan: 1003 (930.8MHz + 16.192kHz) power: 109489.92
Then 'kal -c CHANNELNUM -g 20' for each channel number.
I then get the results below which the average frequency error looks
different for each channel. It is consistent though if I run them multiple
times.
I would have expected less frequency error since I know that it is really
only
about 4Khz off. Or am I not reading the results correctly?
Results seem OK. Do not forget that most BSTs hop around in their allotted
20 Khz channel.
So a result of +/- 20 Khz shouldn't surprise you. Best calibrate against
"anchor" BSTs that are steady.
And yes, recommendation is to calibrate against 3 BSTs and correct for the
average or stddev if you wish...
Nikos
-Rick
Using device 0: Generic RTL2832U OEM
Found Fitipower FC0013 tuner
Exact sample rate is: 270833.002142 Hz
Setting gain: 100.0 dB
kal: Calculating clock frequency offset.
Using E-GSM-900 channel 991 (928.4MHz)
average [min, max] (range, stddev)
+ 21.342kHz [17837, 27433] (9596, 3402.206299)
overruns: 0
not found: 199
average absolute error: -22.988 ppm
Using device 0: Generic RTL2832U OEM
Found Fitipower FC0013 tuner
Exact sample rate is: 270833.002142 Hz
Setting gain: 100.0 dB
kal: Calculating clock frequency offset.
Using E-GSM-900 channel 1002 (930.6MHz)
average [min, max] (range, stddev)
- 11.529kHz [-15293, -5679] (9614, 3640.780273)
overruns: 0
not found: 194
average absolute error: 12.389 ppm
Using device 0: Generic RTL2832U OEM
Found Fitipower FC0013 tuner
Exact sample rate is: 270833.002142 Hz
Setting gain: 100.0 dB
kal: Calculating clock frequency offset.
Using E-GSM-900 channel 1003 (930.8MHz)
average [min, max] (range, stddev)
+ 14.283kHz [9462, 19098] (9636, 4777.044922)
overruns: 0
not found: 126
average absolute error: -15.345 ppm